The invention relates to a method for controlling a friction clutch.
One of the most important requirements for a clutch system is the precise momentum in the proximity of the contact point, because this has essential influence upon the start of crawling, driving, and the shifting comfort.
For precisely this reason the so-called contact point is learned directly in the transmission and saved in software. It is also attempted during the drive operation, whenever possible, to update this contact point and adapt the software.
The contact point can essentially be learned in two different fashions. On the one hand, it can be determined via the balance of the moment during crawling, start of driving, or also during the drive operation, generally via a Kalman filter. On the other hand, the contact point can be determined via a change in rotation of the inactive shaft after disengaging the gears, as shown for example in DE 10 2010 024 941 A1.
In the first case, the determination of the contact point based on the momentum is subject to a plurality of errors influencing the calculation, such as offsets of the engine torque and the dynamics of the clutch and/or the motor, or assumptions regarding the dimensional stability of the clutch. Therefore the contact point can usually be determined only with a precision of 1 mm, which is generally too vague. For this reason, the contact points of all dry duplex clutch systems are commonly determined via a second method based on transmission input shafts.
In the method based on the transmission input shaft, the inactive gear is rapidly disconnected in an inactive clutch that is open and in close proximity of the contact point. This acceleration of the inactive shaft can then be used for the determination of the momentum at the inactive clutch and thus the contact point. The measurement with an open clutch serves as a reference and/or determination of drag moment.
This method is disadvantageous in that constant driving operation is necessary in order to obtain a comparison. In case of traffic jams or driving in urban areas this can hardly be used. Therefore, frequently the momentum-based determination is activated as an “emergency strategy”. However it does not serve for any fine adjustment but rather to prevent major errors.
All of these strategies are based on the fact that the contact point changes only very slowly, noticeable over several 100 km. Rapid changes had to be pilot controlled, in principle. For example, the contact point can be shifted by 2 mm due to temperature. If this is not pilot controlled, after the vehicle was parked and the clutch cooled down, here a jerk or flare may result until the contact point has been learned again. In principle, errors are only learned with a time lag so that a fixed change rate of the contact point will always show an averaging error, see FIG. 1. However, it has shown that new clutch systems tend to settle in the vehicle within the first 100 km upon the initial start of operation of the clutch and here change the contact point by approximately 2 mm. This change can considerably contribute to tip-in strikes and body vibrations in the first kilometers upon engaging the clutch.